1. Field of the Invention
The present invention relates to an optical amplifier applicable to wavelength division multiplexing (WDM) transmission systems, an optical communication system including the optical amplifier, and an optical fiber module constituting a part of the optical amplifier.
2. Related Background Art
In WDM transmission systems including optical amplifiers as optical communication systems, OADM (Optical Add/Drop Multiplexer) and OXC (Optical Cross Connect) have recently been incorporated as structures for separating a part of multiplexed signals by signal channel units or sending out a predetermined wavelength of signals or multiplexed signals to transmission lines by signal channel units in order to improve the reliability and operating efficiency of each system as a whole. In such a WDM system, attention is paid to transitional output signal power fluctuations in optical amplifiers occurring due to changes in the number of channels of inputted signals. Therefore, it is required for optical amplifiers to be controlled such that the output signal power (output signal level) per channel becomes constant even if the input power changes.
As means for suppressing the transitional output signal power fluctuations caused by input signal power fluctuations occurring due to changes in the number of signal channels, high-speed automatic gain control (AGC) has been proposed.
Here, the response speed of AGC is required to become faster as the speed at which the number of signal channels changes is faster, and as the number of the added or dropped signal increases.
As a technique for speeding up AGC, Seo Yeon Park, et al., xe2x80x9cDynamic Gain and Output Power Control in a Gain-Flattened Erbium-Doped Fiber Amplifier,xe2x80x9d IEEE PHOTONICS TECHNOLOGY LETTERS, Vol. 10, No. 6, JUNE 1998, for example, proposes feed-forward control by detecting only the power of input signal (input signal level), and a pumping light power corresponding to the power of input signal is calculated by use of a linear expression, so as to maintain a constant gain. This AGC technique can achieve higher speed more easily as compared with conventional feedback control, whereby a response time of 650 ns is realized in actually developed control circuits.
The inventor has studied the prior art and, as a result, has found a problem as follows. Namely, even if feed-forward control is carried out in a conventional optical amplifier by detecting power fluctuations in input signals, the control starting time will be delayed due to a delay in a control circuit, whereby overshoot may occur by the amount of delay in the control circuit.
If the response time of the control circuit controlling a pumping light source is substantially zero, the power of pumping light outputted from the pumping light source can be regulated at substantially the same time when a power fluctuation in input signals is detected in the vicinity of an input end of an optical amplifier (the fluctuation is detected before amplification), whereby transient power fluctuations in output signals will hardly occur. However, actual control circuits have a response time to a certain extent. Therefore, even if a power fluctuation in input signals is detected in the vicinity of an input end of an optical amplifier (at the point of time indicated by arrow A in FIG. 1), a transient power fluctuation (dynamic gain fluctuation PD) in output signals will occur in an optical amplifier for the duration of response time t, of the control circuit after the actual power fluctuation in input signals is generated as shown in FIG. 1 (see A. K. Srivastava, et al., xe2x80x9cEDFA Transient Response to Channel Loss in WDM Transmission System,xe2x80x9d IEEE PHOTONICS TECHNOLOGY LETTERS, Vol. 9, No. 3, MARCH 1997). Also, Y. Sun et al., xe2x80x9cFast power transients in WDM optical networks with cascaded EDFAs,xe2x80x9d ELECTRONIC LETTERS, Feb. 13, 1997, Vol. 3, No. 4 indicate that transient gain fluctuations accumulate in a system in which optical amplifiers (EDFAs) are connected in a multistage fashion.
However, the change delay time of the pumping light power evaluated in the above-mentioned Srivastava reference with respect to the power fluctuation in input signals is 7 xcexcs or more, whereby it is unknown whether the transient power fluctuation in output signals can fully be suppressed or not if the delay time further approaches zero. Also, it does not mention any difference in response of the optical amplifier depending on the wavelength of pumping light.
In order to overcome the above-mentioned problem, it is an object of the present invention to provide an optical amplifier comprising a structure which can effectively suppress transient output signal power fluctuations (gain fluctuations) occurring due to delays in control, an optical fiber module included in the same, and an optical communication system comprising a structure for effectively restraining the power fluctuations from accumulating.
The optical amplifier according to the present invention is an optical device, employed in a WDM transmission system for transmitting a plurality of channels of signals included in a predetermined signal wavelength band, for amplifying the signals propagating through an optical transmission line included in the WDM transmission system.
In order to overcome the above-mentioned problem, the optical amplifier according to the present invention comprises, at least, a branching device, a light-receiving device, an amplification optical device (amplification optical fiber), a pumping light source, a delay medium, and a control system. The branching device taps part of light including the signals propagating through the optical transmission line. The light-receiving device detects a power fluctuation (level fluctuation) of the part of light tapped by the branching device. The amplification optical fiber is an optical fiber doped with a rare-earth element or the like for amplifying the signals. The pumping light source launches pumping light of a predetermined wavelength into the amplification optical fiber. The delay medium is an optical device, arranged between the branching device and the amplification optical fiber, for delaying the light reaching the input end of the amplification optical fiber from the branching device. The control system monitors, by way of the light-receiving device, the power of the part of light tapped by the branching device, and controls the pumping light source.
Transient power fluctuations in output signals can fully be suppressed if the pumping light power can intentionally be changed at the same time when the fluctuation in input signal level occurring in the vicinity of the input end of the optical amplifier is detected (the amount of change in pumping light power being adjusted according to the amount of fluctuation in input signal power). However, it is unrealistic for the response time from the detection of fluctuation to the power control of pumping light to become zero. Therefore, the present invention utilizes the delay medium, so as to delay the signal input to the amplification optical fiber, thus making it possible to detect the input signal power before amplification.
Preferably, in the optical amplifier according to the present invention, the delay medium has an insertion loss of 3 dB or less. This is because of the fact that it can effectively restrain transient or dynamic power fluctuations from occurring due to changes in the number of signal channels while suppressing an excess degradation of noise figure.
Specifically, it is preferred that the signal delay time given by the delay medium be at least 1 xcexcs but not longer than 50 xcexcs. Preferably, the delay medium includes a single-mode optical fiber having a length of at least 0.2 km but not longer than 10 km. Since the transmission loss of a single-mode optical fiber is usually about 0.2 dB/km, it can minimize the degradation of noise figure in the optical amplifier, and can delay input signals to the amplification optical fiber by a time within the range of 1 xcexcs to 50 xcexcs. As the optical fiber employed in the delay medium, a pure silica core fiber having a low loss and a low bending loss is preferable for allowing the optical amplifier to realize a smaller size.
In the case where the delay medium is longer, the delay medium preferably includes a dispersion-shifted optical fiber in order to restrain the dispersion of the delay medium itself from increasing. The delay medium may also include a dispersion-compensating optical fiber arranged between the single-mode optical fiber and the amplification optical fiber. This is because of the fact that, when a high-speed operation is required in L band (1565 nm to 1620 nm) in the optical amplifier, it will be preferable if the dispersion of the amplification optical fiber is compensated for in order to improve the dispersion tolerance of the optical amplifier.
The inventor has also found that the response characteristic of the optical amplifier depends on the pumping light wavelength. In the case of an optical amplifier employing 0.98 xcexcm as its pumping light wavelength, the signal delay time given by the delay medium must be longer than the response time of the control system from when the signal power fluctuation is detected until when the control of the pumping light source is started. This is because of the fact that transient power fluctuations of an output signal cannot fully be suppressed in the optical amplifier pumped with the pumping light having a wavelength of 0.98 xcexcm even when the pumping light power is changed at substantially the same time when the power of an input signal fluctuates. In the case of the optical amplifier employing 0.98 xcexcm as its pumping light wavelength, the signal delay time given by the delay medium is preferably adjusted such that, in terms of the transient fluctuation of the gain in the optical amplifier caused by a power fluctuation in the input signal with respect to a target gain, the gain yields an overshoot amount and an undershoot amount which are substantially identical to each other.
In the case of an optical amplifier employing 1.48 xcexcm as its pumping light wavelength, it is preferred that the signal delay time given by the delay medium be substantially identical to the response time of the control system from when the signal power fluctuation is detected until when the control of the pumping light source is started. This is because of the fact that transient power fluctuations of the output signal can fully be suppressed in this case unlike the above-mentioned case where the pumping light wavelength is 0.98 xcexcm.
Preferably, the signal delay time given by the delay medium is adjusted so as to become longer than the sampling time of the control system for monitoring the light tapped by the branching device. This aims at making it possible to adjust the pumping light power in conformity to power fluctuations in input signals.
The control system carries out AGC by utilizing a linear expression PP=axc2x7PIN+b (where a and b are constants) which provides a relationship between the power PIN of the part of light tapped by the branching device and the power PP of the pumping light outputted from the pumping light source. This aims at enabling high-speed control by use of feed-forward control. However, even when transient power fluctuations in the output signal are suppressed during the control time t2 as shown in FIG. 1, there is a possibility of a static gain fluctuation PQ occurring if AGC is carried out in a state where the average value (time average) of saturation power of output signals varies between before and after the fluctuation.
Therefore, it is preferred that the optical amplifier according to the present invention further comprise a spectrum analyzer for monitoring a signal distribution within a signal wavelength band at the output end of the amplification optical fiber. Here, the control system detects the signal distribution within the signal wavelength band at the output end of the amplification optical fiber, and updates the constants a and b in the above-mentioned linear expression utilized for AGC.
For effectively suppressing the static gain fluctuation PQ, the control system may update the constants a and b in the above-mentioned linear expression utilized for AGC by using an amplification factor (POUT/PIN) given by the ratio between the amplified optical power POUT at the output end of the amplification optical fiber and the power PIN of the part of light tapped by the branching device.
Preferably, the optical amplifier according to the present invention further comprises a branching device for tapping part of amplified light transmitted through the amplification optical fiber, and a light-receiving device for detecting a power of the light tapped by the branching device. In this case, the control system monitors respective powers of parts of light tapped by the two branching devices disposed upstream and downstream the amplification optical fiber by way of their corresponding light-receiving devices, and controls the pumping light source so as to adjust the pumping light power according to results of detection obtained by these light-receiving devices.
The optical communication system according to the present invention includes a plurality of optical amplifiers at least one of which has a structure identical to that of the optical amplifier comprising the structure mentioned above. Preferably, this optical communication system comprises a structure for enabling at least one of a branching function of tapping part of multiplexed signals propagating through a transmission line by a signal channel unit, and an inserting function of sending out a predetermined wavelength of signals or multiplexed signals to a transmission line by a signal channel unit, e.g., OADM or OXC.
The optical amplifier according to the present invention preferably includes an optical fiber module having respective terminals to be connected to the light-receiving device, pumping light source, and optical transmission line so as to reduce the insertion loss of the optical amplifier. The optical fiber module comprises a single-mode optical fiber to be fusion-spliced to the optical transmission line, a delay fiber acting as the delay medium to be fusion-spliced to the single-mode optical fiber, an amplification optical fiber to be fusion-spliced to the delay fiber, a first fiber coupler for tapping part of light including a signal having reached the optical amplifier before being fed to the delay fiber, and a second fiber coupler for supplying the pumping light from at least one of the input and output ends of the amplification optical fiber.
Specifically, the single-mode optical fiber has a first end to be fusion-spliced to the output end of the optical transmission line through which a plurality of channels of signals included in a predetermined signal wavelength band propagate, and a second end opposing the first end; and constitutes a part of the optical transmission line. The delay fiber has a first end fusion-spliced to the second end of single-mode optical fiber, and a second end opposing the first end. The amplification optical fiber has a first end fusion-spliced to the second end of delay fiber, and a second end opposing the first end. The first fiber coupler is arranged near a fused part between the second end of single-mode optical fiber and the first end of delay fiber. Preferably, the second fiber coupler is arranged near a fused part between the second end of delay fiber and the first end of amplification optical fiber, or near the second end of amplification optical fiber.
The present invention will be more fully understood from the detailed description given hereinbelow and the accompanying drawings, which are given by way of illustration only and are not to be considered as limiting the present invention.
Further scope of applicability of the present invention will become apparent from the detailed description given hereinafter. However, it should be understood that the detailed description and specific examples, while indicating preferred embodiments of the invention, are given by way of illustration only, since various changes and modifications within the spirit and scope of the invention will be apparent to those skilled in the art from this detailed description.